1. Field of the Invention
This invention relates to the field of integrated circuit fabrication, and more particularly to methods and apparati for improving atomic layer deposition and other chemical reaction techniques.
2. Description of Prior Art
Thin film deposition is common in the fabrication of semiconductor devices and many other useful devices. Chemical vapor deposition (CVD) techniques utilize chemically reactive molecules that react on a substrate to deposit a desired film. Reactants in CVD processes comprise volatile molecules that can be practically delivered, in the gas phase, to react on the substrate to deposit a desired film.
Conventional CVD is practiced in the art by a variety of techniques. Desired thin film properties and cost-effective operational parameters influence the choice of equipment, precursor composition, pressure range, temperature, and other variables. Common to most CVD techniques is the application of a well-controlled flux of one or more molecular precursors into the CVD reactor. A substrate is kept at a well-controlled temperature under well-controlled pressure conditions to promote chemical reaction between the molecular precursors concurrent with efficient desorption of by-products. The chemical reaction is allowed to proceed to deposit the desired thin film with a desired film thickness.
Optimum CVD performance directly correlates with the ability to achieve and sustain steady-state conditions of flux, temperature, and pressure throughout the process, in which unavoidable transients are suppressed or minimized. CVD has provided uniform and conformal coatings with reproducible thickness and exceptional quality.
Nevertheless, as device density increases and device geometry becomes more complicated in integrated circuit devices, the need for thinner films with superior conformal coating properties has approached the limits of conventional CVD techniques and new techniques are needed. An emerging variant of CVD, atomic layer deposition (“ALD”) offers superior thickness control and conformality for advanced thin film deposition.
ALD is practiced by dividing conventional thin-film deposition processes into single atomic layer deposition steps that are self-terminating and deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically equals about 0.1 molecular monolayer to 0.5 molecular monolayer. The deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and eliminates the “extra” atoms originally included in the molecular precursor.
In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. For example, a metal precursor molecule, MLx, comprises a metal element, M (e.g., M=Al, W, Ta, and Si), that is bonded to atomic or molecular ligands, L. The metal precursor reacts with the substrate. This ALD reaction occurs only if the substrate surface is prepared to react directly with the molecular precursor. For example, the substrate surface typically is prepared to include hydrogen-containing ligands, AH, that are reactive with the metal precursor. The gaseous precursor molecule effectively reacts with all the ligands on the substrate surface, resulting in deposition of an atomic layer of the metal: substrate-AH+MLx→substrate-AMLx−1+HL, where HL is a reaction by-product. During the reaction, the initial surface ligands, AH, are consumed, and the surface becomes covered with L ligands, which cannot further react with metal precursor MLx. Therefore, the reaction self-terminates, or “saturates”, when all the initial AH-ligands on the surface are replaced with AMLx−1 species. The resulting substrate-AMLx−1 surface is an ALD intermediate surface, which is essentially covered with the L ligands.
The reaction stage is typically followed by an inert-gas purge stage that eliminates the metal precursor from the chamber prior to the separate introduction of the other precursor.
A second molecular precursor then is used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and redepositing AH ligands. In this case, the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H2O, NH3, H2S). The reaction, substrate-ML+AHY→substrate-M−AH+HL, (here, for the sake of simplicity, the chemical reactions are not balanced) converts the surface back to being AH-covered. The desired additional element, A, is incorporated into the film and the undesired ligands, L, are eliminated as volatile by-product. Once again, the reaction consumes the reactive sites (this time, the L-terminated sites) and self-terminates (saturates) when the reactive sites on the substrate are entirely depleted. A different, second ALD intermediate surface is thereby created. In the simple example given here, the second intermediate ALD surface is similar to the initial surface, where the initial surface represents the surface prior to the introduction of the metal precursor.
Typically, the second molecular precursor then is removed from the deposition chamber by flowing inert purge-gas in a second purge stage.
This sequence of surface reactions and precursor-removal that restores the substrate surface to its initial reactive state is a typical ALD deposition cycle. Restoration of the substrate to its initial condition is a key aspect of ALD. It implies that films can be layered down in equal metered sequences that are all identical in chemical kinetics, deposition per cycle, composition, and thickness. Self-saturating surface reactions make ALD insensitive to transport nonuniformity. This transport nonuniformity may pertain either to the engineering and the limitations of the flow system or could be related to surface topography (i.e., deposition into three-dimensional, high-aspect ratio structures). Nonuniform flux of chemicals can only result in different completion times at different areas. However, if each of the reactions is allowed to complete on the entire substrate surface, the different completion kinetics bear no penalty. This is because the areas that are first to complete the reaction self-terminate the reaction, while the rest of the area on the surface is able to complete the reaction, self-terminate, and essentially catch up.
Efficient practice of ALD requires an apparatus capable of changing the flux of chemicals from MLx into AHy abruptly and fast. Furthermore, the apparatus must be able to carry this sequencing efficiently and reliably for many cycles to facilitate cost-effective coating of many substrates. Typically, an ALD process deposits about 0.1 nanometer (nm) of a film per ALD cycle. A useful and economically feasible cycle time must accommodate a thickness in a range of about from 3 nm to 30 nm for most semiconductor applications, and even thicker films for other applications. Industry throughput standards dictate that substrates be processed in 2 minutes to 3 minutes, which means that ALD cycle times must be in a range of about from 0.6 seconds to 6 seconds. Multiple technical challenges have so far prevented cost-effective implementation of ALD systems and methods for manufacturing of semiconductor devices and other devices.
Generally, an ALD process requires alternating in sequence the flux of chemicals to the substrate. A representative ALD process, as discussed above, requires four different operational stages:
1. MLx reaction;
2. MLx purge;
3. AHy reaction; and
4. AHy purge.
Given the need for short cycle times, chemical delivery systems suitable for use in ALD must be able to alternate incoming molecular precursor flows and purges with sub-second response times. Also, if significant flow nonuniformities exist, these can be overcome through the self-terminating nature of the chemical reactions by increasing the reaction-stage time to the time dictated by areas that are exposed to the smallest flux. Nevertheless, this necessarily degrades throughput since cycle times increase correspondingly.
In order to minimize the time that an ALD reaction needs to reach self-termination, at any given reaction temperature, the flux of chemicals into the ALD reactor must be maximized. In order to maximize the flux of chemicals into the ALD reactor, it is advantageous to introduce the molecular precursors into the ALD reactor with minimum dilution of inert gas and at high pressures. On the other hand, the need to achieve low short cycle times requires the rapid removal of these molecular precursors from the ALD reactor. Rapid removal in turn dictates that gas residence time in the ALD reactor be minimized. Gas residence times, τ, are proportional to the volume of the reactor, V, the pressure, P, in the ALD reactor, and the inverse of the flow, Q, τ=VP/Q. Accordingly, lowering pressure (P) in the ALD reactor facilitates low gas residence times and increases the speed of removal (purge) of chemical precursor from the ALD reactor. In contrast, minimizing the ALD reaction time requires maximizing the flux of chemical precursors into the ALD reactor through the use of a high pressure within the ALD reactor. In addition, both gas residence time and chemical usage efficiency are inversely proportional to the flow. Thus, while lowering flow will increase efficiency, it will also increase gas residence time.
Conventional ALD apparati have struggled with the trade-off between the need to shorten reaction times and improve chemical utilization efficiency, and, on the other hand, the need to minimize purge-gas residence and chemical removal times. Certain ALD systems of the prior art contain chemical delivery manifolds using synchronized actuation of multiple valves. In such systems, satisfactory elimination of flow excursions is impossible because valve actuation with perfect synchronization is itself practically impossible. As a result, the inevitable flow excursions are notorious for generating a backflow of gas that leads to adverse chemical mixing. Improved methods and systems of ALD using synchronous modulation of flow and draw (“SMFD”) are disclosed in co-owned and copending U.S. patent application Ser. No. 10/347,575, filed Jan. 17, 2003, which is hereby incorporated by reference.
Practical implementation of conventional ALD in commercial manufacturing applications is also limited by a scarcity of suitable chemical sequences, for example, an MLx and AHy sequence (and sometimes by sequences requiring more than two chemicals), that enable the desired deposition with adequate speed and with adequate results. Chemical sequences suitable for conventional ALD are not generic. ALD precursors should be stable at the reaction temperature. Self-decomposition of ALD precursors at the reaction temperature prevents self-limitation, or saturation. ALD precursors should react efficiently with the intermediate surface that is created by the previous chemical reaction in an ALD cycle. For example, MLx molecules should react efficiently with the surface terminated with AH species. If the reactions are not efficient, reaction times must be extended to allow for the reactions to occur. Increasing reaction times limits the throughput that can be achieved. In addition, unreacted ligands, for example, the -AH and -ML ligands, can degrade the purity of a film by inclusion of undesired elements into the film, such as H and the elements composing the ligand L in the above. Furthermore, a molecular ALD precursor, such as MLx, should react only with some of their ligands, while other ligands should stay attached to the surface. For example, -ML should saturate the surface in a self-terminating reaction to serve effectively as the reactive site on the immediate surface available to react with the other molecular precursor, for example, AHy. Reaction by-products should be volatile. For example, the HL reaction by-product, which evolves both from the reaction of AHy on -ML surfaces and from the reaction of MLx on -AH surfaces, must be volatile. Finally, the surface species that are left on an intermediate surface following the completion and self-termination of ALD reactions should be stable, with no or minimized desorption during the time that is practically necessary to remove the excess precursor and reaction by-products and the time that it takes to complete the next surface reaction in sequence. For example, the -ML surface termination sites should be stable during the time that it takes to sweep the MLx molecules out of the reaction chamber and the time necessary for the next reaction with AHy to complete.
These requirements for ALD precursor selection (e.g., pairs, triplets) impose limitations on the films that can be produced in manufacturing-grade ALD processes, due to the limitations on reaction rates and film purity caused by chemical sequences that do not meet these requirements. Accordingly, although many different types of films have been deposited using ALD processes conducted in research settings, very few of these films (and related processes) are suitable for use in commercial ALD manufacturing. Unfortunately, the limited number of films suitable for use in commercial ALD manufacturing applications does not include many of the films having potential commercial importance. For example, an adequate ALD precursor combination has not been found in the prior art for single-element metal and semiconductor films needed for semiconductor applications, such as titanium, tantalum, copper, silicon, and tungsten. Likewise, most nitride films, such as TiN, TaNx, WNx, and Si3N4, have not been demonstrated with precursor-combinations that are adequate for cost-effective production. Also, many dielectric materials that are desired in the manufacture of semiconductor and other devices have not been demonstrated with adequate precursor combinations. Accordingly, there is a need to develop a greater variety of precursor combinations that are suitable for commercial ALD manufacturing applications.
In addition to the aforementioned limitations, ALD has another serious fundamental limitation. Unlike CVD reactions (usually steady-state) that are continuous and non-saturating, ALD reactions follow kinetics of molecule-surface interaction. The kinetics of molecule-surface reactions depend on the individual reaction rate between a molecular precursor and a surface reactive site and on the number of available reactive sites. As the reaction proceeds to completion, the surface is converted from being reactive to non-reactive. As a result, the actual process rate is slowing down during the deposition. Accordingly, ALD completion rates, dN/dt, are proportional to the number of reactive sites, dN/dt=−kN, where N is the number of reactive sites and k is the (single site) reaction rate. Elimination of the reactive sites for reaction follows an exponential time dependence N(t)=N0exp(−kt). Accordingly, the “self-terminating” reactions essentially never self-terminate (as they would require an infinite time to terminate because the rate is exponentially decreasing). This fundamental property of molecule-surface kinetics was named after the scientist Langmuir, and is well known in the art of surface science. The limitations of Langmuirian kinetics present a significant limitation on overall throughput in conventional ALD.
As noted above, the limitations described by Langmuirian kinetics dictate that the surface is never “completely” reacted. If the surface is not completely reacted, there are necessarily leftover undesired elements in the film. For example, if an MLx reaction does not totally consume the surface -AH sites, then the film incorporates H. Likewise, if the AHy reaction is not carried to completion, undesired L-incorporation results. The quality of a film depends on impurity levels. Thus, ALD suffers from a throughput-quality tradeoff. Namely, to achieve greater throughput, it is generally assumed that quality must be sacrificed, and vice-versa. This throughput-quality tradeoff is of particular concern because it carries an exponential throughput penalty to attain a linear reduction of impurity levels.
Most critical applications of ALD films, particularly of semiconductor films, include stringent specifications regarding impurity levels. Accordingly, to achieve low impurity levels, ALD reactions typically must be conducted beyond 99% saturation. As noted above, however, Langmuirian kinetics dictate that conducting an ALD reaction up to or beyond 99% saturation typically causes a serious reduction in throughput.
Elevating ALD process temperatures potentially overcomes the limitations posed by Langmuirian kinetics since ALD reactions are thermally activated. Nevertheless, in most cases, ALD process temperatures are practically limited by one or more of the following factors: 1) device manufacturing integration becomes more difficult at increased process temperatures; 2) instability of surface ligands and molecular precursors generally increases with process temperature; and 3) deposition per cycle inherently decreases with increased process temperature for most known ALD processes, thus resulting in the need to run more cycles to deposit the film up to the target thickness. As a result, many specific ALD precursor combinations and potential ALD films are rendered inadequate for cost-effective commercial ALD manufacturing applications. Accordingly, there is a need for ALD methods and apparati that enable a generic approach for ALD implementations, and that allow fabrication of a larger range of ALD films at commercially feasible throughput and quality levels. Likewise, there is a need for ALD methods and apparati that can surmount the Langmuirian limitations and enhance reaction kinetics without sacrificing film purity.
Despite the limitations of conventional ALD, ALD films have the potential to provide a number of commercial manufacturing advantages. For example, ALD films have unique pinhole free and low stress advantages. Accordingly, ALD films are ideal for device passivation and encapsulation applications. Much thinner encapsulation films can be realized by ALD than with conventional encapsulation processes. Such thinner encapsulation films are advantageous for minimized alteration of device performance. For example, it is desirable to encapsulate display and optical devices by very thin films that minimize impact on light output. However, material and process constraints of many devices limit many passivation and encapsulation techniques for such devices to process temperatures not exceeding 200° C., and sometimes to less than 100° C. In addition, some display and optical devices, for example, Organic Light Emitting Diode (OLED) display devices, are extremely sensitive to moisture and oxidizing conditions. The organic materials commonly used in OLEDs are particularly susceptible to damage caused by exposure to the ambient atmosphere, as well as to reactions of organic compounds with electrode materials. Furthermore, the metals typically utilized as OLED cathodes are highly reactive with oxygen and water and may be negatively affected by oxidation. OLED device encapsulation and protection from moisture and oxygen is currently accomplished by glass windows and vacuum sealing techniques that are costly and cumbersome. Accordingly, there is a need to develop a commercially feasible thin film encapsulation method and apparatus for use in OLED manufacturing that meets the above-mentioned needs.
In particular, there is a need for ALD methods with a generic approach that can increase reaction rates, that are fast and efficient even at low temperatures, and that allow flexibility in the choice of metal ALD precursors.